Rear brake caliper siezing or dragging?

["Colin Barfoot" <Colin_Barfoot@msn.com>]

Qlisters please assist I need to know by tommorrow AM if possible to order the 
correct parts...
On the way home from MT Washington and my vacation following my rear caliper 
siezed or really just dragged enough to wear away the pads completely on the 
passenger side rear.  My mechanic said I had a siezed caliper (it was about 4 
years since I had the calipers rebuilt so I said ok.  I ordered 2 new rotrs 
pads and sent both calipers to be rebuilt (since the bleeder valves had been 
broken off earlier this year at the dealers...
I just had my new rebuilt calipers put on Wednesday and then the car was taken 
for a test drive after the first application or so the right rear caliper was 
siezed on again! ( actually the mechanic said he could only just feel it 
againt my powerful engine...(thanks SK(R)) This was the same one which had 
failed before???!!  
My understanding is (correct if wrong) :The pistons can normally just be 
depressed enough to put in the pads on, to depress further requires screwing 
in the piston.
 The mechanic said that on the caliper which siezed on he could depress the 
piston manually all the way.  Also with the system pressurized he could still 
depress the one caliper (the one siezed on) but the other one (which worked 
fine) could not be depressed.
I sent the siezed on caliper back to the supplier and it will be replaced with 
a different one, but I'm wondering (due to the recent thread on the RR caliper 
dragging with a stuck MC if thats what I have too.  I'll try at least to order 
a new MC tommorrow.  
On the MC does anyone know if you can just order the larger one from the 
wagon(or a later Quattro?) will it fit/work ok?
Opinions?
Thanks,
Colin
86 5000 CD Quattro
307,000 km (off the road for > 2 weeks now!!!)